1. Field of the Invention
This invention relates to an objective lens, to an optical pickup device comprising such an objective lens and also to an optical disk device comprising such an optical pickup device.
2. Related Background Art
A number of different types of optical recording medium such as optical disks have been proposed along with optical pickup devices adapted to write information signals to and read information signals from such an optical recording medium. There have also been proposed a number of different optical disk devices comprising an optical pickup device and adapted to record information signals to and reproduce information signals from an optical disk that is used as optical recording medium.
An optical pickup device is provided with a semiconductor laser operating as light source. The flux of light emitted from the semiconductor laser is converged onto the signal recording surface of the optical recording medium by means of an objective lens. Then, the optical pickup device detects the flux of light reflected by the signal recording surface of the optical recording medium by means of its optical detector.
Therefore, the optical pickup device can read any of the information signals recorded on the optical recording medium on the basis of the output of the optical detector and write information signals on the optical recording medium by irradiating the optical recording medium with a flux of light.
Meanwhile, efforts have been paid to reduce the diameter of the light spot formed on the optical recording medium as a result of the convergence of the flux of light irradiating the optical recording medium by increasing the numerical aperture (NA) of the objective lens in order to raise the density of recording information signals on the optical recording medium.
However, as the numerical aperture of the objective lens is increased, the power of the lens is also increased to make it difficult to design the imaging optical system of the optical pickup device so as to favorably correct the aberration of the optical system and maintain the corrected spherical aberration in the environment that is changing incessantly particularly in terms of temperature.
Thus, it has been highly difficult to achieve a numerical aperture greater than 0.5 in the case of the objective lens of a finite system. The term of xe2x80x9cthe objective lens of a finite systemxe2x80x9d as used herein refers to an objective lens adapted to receive a divergent flux of light emitted from a spot light source separated from it by a finite distance and converge the flux of light onto the signal recording surface of an optical recording medium.
In the case of the objective lens of an infinite system where the objective lens is adapted to receive a parallel flux of light and converge it onto the signal recording surface of an optical recording medium, there have been known those having a numerical aperture greater than 0.5. However, when using the objective lens of an infinite system, it is difficult to downsize the optical pickup device because a collimator lens has to be arranged between the objective lens and the light source in order to transform the divergent flux of light emitted from the light source into a parallel flux of light.
In view of the above identified circumstances, it is therefore an object of the present invention to provide an objective lens having a numerical aperture greater than 0.5 while maintaining the favorably corrected spherical aberration in any environment that is changing incessantly particularly in terms of temperature and adapted to downsizing the optical pickup device comprising it. Another object of the present invention is to provide an optical pickup device comprising such an objective lens and an optical disk device comprising such an optical pickup device.
According to the invention, the above objects are achieved by providing an objective lens having a numerical aperture greater than 0.5 and comprising a single objective lens adapted to a finite system, said objective lens having first and second aspherical surfaces;
a hologram being formed on at least one of said aspherical surfaces, the light of the positive 1st order or that of the negative 1st order of the hologram being subjected to optimal correction of spherical aberration under actual operating conditions;
when using a semiconductor laser as light source, the change in the spherical aberration caused by the change in the refractive index arising as a result of the change in the environmental temperature of the medium between the first surface and the second surface being substantially offset by the change in the spherical aberration of the hologram attributable to the change in the oscillation wavelength of the semiconductor laser of the light source caused by the change in the environmental temperature.
In another aspect of the invention, there is provided an optical pickup device comprising:
a semiconductor laser operating as light source;
an objective lens for converging the flux of light emitted from the semiconductor laser onto the signal recording surface of an optical recording medium; and
a photodetector for detecting the flux of light converged onto the signal recording surface by the objective lens and reflected from said signal recording surface;
said objective lens having a numerical aperture greater than 0.5 and comprising a single objective lens adapted to a finite system, said objective lens having first and second aspherical surfaces;
a hologram being formed on at least one of said aspherical surfaces, the light of the positive 1st order or that of the negative 1st order of the hologram being subjected to optimal correction of spherical aberration under actual operating conditions;
when using a semiconductor laser as light source, the change in the spherical aberration caused by the change in the refractive index arising as a result of the change in the environmental temperature of the medium between the first surface and the second surface being substantially offset by the change in the spherical aberration of the hologram attributable to the change in the oscillation wavelength of the semiconductor laser of the light source caused by the change in the environmental temperature.
In a further aspect of the invention, there is provided an optical disk device comprising:
a rotary drive mechanism for holding an optical disk and driving it to rotate; and
an optical pickup device for recording information signals on or reproducing information signals from the optical disk driven to rotate by said rotary drive mechanism;
said optical pickup device including:
a semiconductor laser operating as light source;
an objective lens for converging the flux of light emitted from the semiconductor laser onto the signal recording surface of an optical recording medium; and
a photodetector for detecting the flux of light converged onto the signal recording surface by the objective lens and reflected from said signal recording surface;
said objective lens having a numerical aperture greater than 0.5 and comprising a single objective lens adapted to a finite system, said objective lens having first and second aspherical surfaces;
a hologram being formed on at least one of said aspherical surfaces, the light of the positive 1st order or that of the negative 1st order of the hologram being subjected to optimal correction of spherical aberration under actual operating conditions;
when using a semiconductor laser as light source, the change in the spherical aberration caused by the change in the refractive index arising as a result of the change in the environmental temperature of the medium between the first surface and the second surface being substantially offset by the change in the spherical aberration of the hologram attributable to the change in the oscillation wavelength of the semiconductor laser of the light source caused by the change in the environmental temperature.
As described above, an objective lens according to the invention comprises a single objective lens adapted to a finite system, said objective lens having first and second aspherical surfaces, a hologram being formed on at least one of said aspherical surfaces, the light of the positive 1st order or that of the negative 1st order of the hologram being subjected to optimal correction of spherical aberration under actual operating conditions;
When using a semiconductor laser as light source, the change in the spherical aberration caused by the change in the refractive index arising as a result of the change in the environmental temperature of the medium between the first surface and the second surface is substantially offset by the change in the spherical aberration of the hologram attributable to the change in the oscillation wavelength of the semiconductor laser of the light source caused by the change in the environmental temperature.
Thus, the present invention provides an objective lens having a numerical aperture greater than 0.5 while maintaining the favorably corrected spherical aberration in any environment that is changing incessantly particularly in terms of temperature and adapted to downsizing the optical pickup device comprising it. According to the invention, there are also provided an optical pickup device comprising such an objective lens and an optical disk device comprising such an optical pickup device.